The invention relates to an electric lamp provided with a ceramic discharge tube and a current inlead consisting of two or more elementary filaments, expediently wire, passing through the ceramic closing member. There is at least one elementary filament, expediently a wire, formed as an auxiliary electrode in the tube.
U.S. Pat. No. 3,243,635 discloses, electric lamps provided with a ceramic discharge tube which is closed on both ends by means of a metal disc serving simultaneously as a current inlead.
In another known electric discharge lamp, the tube is closed on both sides with a ceramic closing member, while the electrode is connected to the outer current lead via a separate current inlead. In the electric lamps with a ceramic discharge tube where the bulbs are closed by a ceramic closing member, soldering of the current inlead into the closing member is a problem due to the differences of the coefficients of thermal expansion.
U.S. Pat. No. 3,660,539 describes an electric discharge lamp wherein the current inlead is formed by a metal wire which is soldered into the bore of the closing member by using a vitreous solder. In order to be able to reduce the difference of the coefficients of thermal expansion, a current inlead made of niobium is used for the alumina- ceramic closing member.
The increased output of the electric discharge lamps, however, requires the delivery of a higher current quantity to the electrodes arranged inside at the two ends of the bulb. Accordingly, the cross-section of the current inlead has to be increased in proportion to the increased current quantity.
As a result of the increase of the cross-section of the current inlead, the heat conduction coming from the direction of the discharge space is also increased within the discharge tube. The "thermal shock" which, in the course of ignition, passes suddenly through the large cross-section current inlead causing a inconsiderable heat-drop with resultant damage and cracks in the ceramic closing member and at the glued, soldered surfaces of the ceramic closing member and metal current inlead. The foregoing phenomenon becomes more intensive as the current passing through the current inlead increases and the cross-section of the current inlead is increased.
The above phenomenon is well known and several methods were tried in efforts to avoid the detrimental stresses and the cracks resulting from it both at the glued or soldered surfaces of the metal current inlead and the ceramic closing member.
U.S. Pat. No. 3,363,134 discloses a method wherein compensation for the rise of the detrimental stresses is made by applying a thin-wall tube made of niobium; in this case an effort was made to reduce the detrimental dilatation effect resulting from the high temperatures arising upon ignition by utilizing the elasticity of the thin-wall tube.
U.S. Pat. No. 3,942,642 discloses a method wherein compensation is effected by lengthening the part of the current inlead connected to the electrode via the ceramic closing member, within the closing member with a helical form bent around the axis of the lamp, whereby the effect of the electrical shock can be delayed.
Both of the above structures have several drawbacks. One of the drawbacks of the device of U.S. Pat. No. 3,363,134 is that the tube-shaped current inlead only reduces the dilatation effect resulting from the thermal shock, but it is unable to eliminate it. Another drawback is that the application of the niobium tube increases production costs of the lamp to such an extent, that low costs cannot be ensured, as the price of the tube amounts to about hundredfold of the dense wire per unit of weight. Accordingly, this device is disadvantageous not only from the technical point of view of, but also of costs.
The drawback of the device of U.S. Pat. No. 3,992,664 is that the structural size of the helically bent current inlead within the lamp can be increased only within certain limits, since it is restricted by the inner size of the ceramic discharge tube of the lamp, and the mutual distance of the electrodes. As a consequence, the effect of the thermal shock cannot be entirely compensated. Simultaneously bending to the helical form and formation of the of the special lug needed for the connection to the electrode make the device difficult to produce.
The object of this invention is to provide an electric lamp with a ceramic discharge tube that does not have the drawbacks enumerated above. A further object of this invention is to provide such a lamp that can stop the detrimental dilatation effect resulting from thermal shocks, even with an enlarged cross-section of the current inlead, which is the structure of the high-output lamps and which is easily and economical to produce, yet is safe to operate.